Development history and activity characteristics of typical debris flows in the Grand Bend of the Yarlung Zangbo River since the Holocene
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摘要:
雅鲁藏布江大拐弯附近晚更新世末次冰期—全新世发育多期次泥石流, 组合形成了现代大规模扇形堆积体。以派镇蹦嘎沟泥石流为例, 采用地面调查、钻孔及14C测年等方法, 研究泥石流形成年代序列、堆积深度、冲出范围等特征, 分析结果表明: 现代蹦嘎沟依然有小规模的支沟泥石流发育且广泛堆积于沟道内, 现存堆积扇区域尚未发现泥石流堆积; 距今8500年左右为蹦噶沟全新世泥石流活跃期, 单期次累积堆积深度约10.9 m; 滨湖浅水相沉积(河流相)形成的浅灰色粉细砂中的两处碳样表明雅鲁藏布江现代河床在40~100年左右沉积深度约0.4 m, 年平均沉积速率4~10 mm; 海拔2906.1~2896.7 m及2849.4~2848.2 m处钻孔依次揭露厚度为9.4 m和1.2 m饼状青灰色粉质黏土, 推测发生两次堵江事件。上述结果可为该区域全新世以来泥石流活动性特征研究提供参考。
Abstract:Multi-period debris flows have been developed in the last glacial period of the late Pleistocene-Holocene near the Grand Bend of the Yarlung Zangbo River in southeast Tibet, which combined to form a modern large-scale fan-shaped accumulation. The debris flows in the Bangga gully, Pai Town, were explored by ground survey, borehole, and 14C dating methods to investigate the chronological sequence of formation, accumulation depth, and outrush range. The analysis results show that there are still small-scale debris flows in the tributaries of the Bengga gully, and they are widely accumulated in the channel, but no debris flow accumulation has been found in the existing accumulation fan area. The Holocene debris flows in the Bunga gully were active around 8500 years ago, and the cumulative accumulation depth of a single period is about 10.9 m. The two carbon samples in the light gray silt sand formed by the shallow lake facies (fluvial facies) show that the modern riverbed of the Yarlung Zangbo River was deposited at a depth of about 0.4 m in 40 to 100 years, and the annual average deposition rate was about 4~10 mm. The boreholes at 2906.1~2896.7 m and 2849.4~2848.2 m above sea level reveal a thickness of 9.4 m and 1.2 m cake-like bluish-gray clay in turn. It is assumed that two river-blocking events occurred. The above results could provide a reference for the study of the debris flow activity characteristics since the Holocene in this region.
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1. 研究目的（Objective）
近年来，西昆仑大红柳滩地区先后新发现白龙山锂铍多金属矿、509道班西锂铍矿、大红柳滩南锂矿等一系列锂铍矿床。现有矿床勘探成果显示，大红柳滩地区有望成为一个世界级巨型锂多金属矿集区。前人主要从锆石、铌钽铁矿等矿物开展了大红柳滩地区伟晶岩型锂矿床的成矿时代测定（220~200 Ma），然而伟晶岩的高精度定年是一个难题，需要运用不同的同位素定年进行综合研究。509道班西锂铍稀有金属矿品位高，规模大，Li2O资源量可突破百万吨，是大红柳滩地区最具代表性的矿床。挑选509道班西锂铍稀有金属矿床含矿及不含矿伟晶岩中白云母开展40Ar/39Ar同位素测年，获得精确的测年年龄，为限定区域成矿时代提供数据支撑。
2. 研究方法（Methods）
白云母单矿物挑选由廊坊市宇恒矿岩技术服务有限公司完成。挑选出纯度 > 99%的白云(40 mg) 先用超声波清洗干净后，被封进石英瓶中送核反应堆中接受中子照射，总时间为2160 min，同期接受中子照射的监控标准样ZBH-25黑云母标准年龄为(132.7±1.2) Ma，K含量为7.6%。使用石墨炉升温加热，每一个阶段加热10 min，净化20 min。质谱分析是在多接收稀有气体质谱仪GVHelixMC上进行的，每个峰值均采集20组数据。所有的数据在回归到时间零点值后再进行质量歧视校正、大气氩校正、空白校正和干扰元素同位素校正。用ArArCALC程序计算坪年龄及正、反等时线。坪年龄误差以2σ给出。
3. 研究结果（Results）
白云母测试样品分别取自509道班西Ⅰ、Ⅱ号脉群Ⅰ-19、Ⅱ-1矿体及石英闪长岩岩体中含电气石伟晶岩，样品编号为TC11-1、TC37-1、DLT004、DLT006，采样位置见图 1。宏微观特征显示，白云母与锂辉石形成共生组合，表明两者同时期形成，因此白云母形成年龄也代表了成矿时代。
图 1. 509道班西锂铍矿地质简图、白云母坪年龄谱图和反等时线年龄图1—第四系冲洪积相; 2—巴颜喀拉山群上组下段; 3—巴颜喀拉山群中组上段; 4—晚石炭世石英闪长岩; 5—含黑电气石花岗伟晶岩; 6—晚石炭世二长花岗岩; 7—含石榴子石白云母黑电气石伟晶岩脉; 8—含锂辉石伟晶岩脉; 9—右旋断层; 10—采样点; SKT—南昆仑地体; TST—甜水海地体; AKTS—阿克塔什锂矿床; DHLT—大红柳滩南锂矿床; BLS—白龙山锂矿床; Sp—锂辉石; Ab—钠长石; Ms—白云母; Qz—石英; Pl—斜长石Figure 1. Geological sketch and plateau age and inverse isochron age of Pegmatite type rare metal lithium deposit in western 509 Daoban1-Quaternary alluvial-diluvial facies; 2-Lower Member of Upper Formation of Bayan Khara Mountain Group; 3-Upper Member of Middle Formation of Bayankhara Mountain Group; 4-Late Carboniferous quartz diorite; 5-Black tourmaline granitic pegmatite; 6-Late Carboniferous monzogranite; 7-Garnet-bearing muscovite black tourmaline dikes; 8-Spodumene viritic dikes; 9-Dextral fault; 10-Sampling point; SKT-South Kunlun terrane; TST-Tian shui hai Terrane; AKTS-Aktash lithium deposit; DHLT-Dahongliutan South Lithium deposit; BLS-Bailongshan Lithium deposit; Sp-Spodumene; Ab-Albite; Ms-White mica; Qz-Quartz; Pl -Plagioclase测试数据见表 1。所有样品皆表现出稳定的年龄图谱，坪年龄和等时线及反等时线年龄结果一致, 数据可靠，平均后得到白云母的形成年龄：TC11-1为(184.59±1.81)Ma、TC37-1为(179.36±1.84) Ma、DLT004为(182.79 ± 1.85) Ma、DLT006为(181.95 ± 1.89) Ma（图 1）。
表 1. 伟晶岩脉中白云母40Ar/39Ar同位素分析结果Table 1. 40 Ar/39 Ar isotopic data of muscovite from pegmatite vein
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4. 结论（Conclusion）
白云母40Ar/39Ar定年结果显示，509道班西锂辉石伟晶岩形成于185~179 Ma，首次精确限定了该区的成矿时代。作为西昆仑大红柳滩地区重要的伟晶岩型锂矿，揭示其形成时代将为西昆仑—甜水海地区找寻同期花岗伟晶岩型锂矿提供借鉴。
5. 基金项目（Fund support）
本文为国家重点研发计划“我国西部伟晶岩型锂等稀有金属成矿规律与勘查技术”（2021YFC2901900）、“第二次青藏高原综合科学考察研究”项目“西昆仑—喀喇昆仑稀有金属资源现状与远景评估”（2019QZKK080201）联合资助的成果。
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表 1 测年取样样品及其位置统计表
Table 1. Statistical table of the samples for dating and their locations
样品位置及取样编号 样品照片 
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表 2 漂木样品测年结果
Table 2. Dating results of the driftwood samples
样品编号 埋深/m 放射性碳年龄 公历校正 测年概率/% ZK135-1 12.6 7120+/－30 a B.P. 6062—5977 cal.B.C. 76.0 5948—5919 cal.B.C. 19.4 ZK135-4 13.0 7220+/－30 a B.P. 6100—6011 cal.B.C. 76.7 6219—6135 cal.B.C. 18.7 ZK135-3 38.5 7810+/－30 a B.P. 6696—6568 cal.B.C. 93.4 ZK139-2 61.0 9370+/－40 a B.P. 8753—8547 cal.B.C. 93.7 ZK139-1 61.8 9420+/－30 a B.P. 8792—8622 cal.B.C. 95.4
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表 3 蹦嘎沟泥石流物源分布及时间变化特征
Table 3. Source distribution and temporal variation characteristics of the sources in the Bengga gully
编号 类型 面积/m2 厚度/m 体积/×104 m3 影像日期 BH01 崩滑物源 95734 4.21 40.29 2007-9-17 BH02 崩滑物源 26254 3.13 8.21 2007-9-17 BH03 崩滑物源 44733 3.54 15.81 2007-9-17 BH04 崩滑物源 51883 3.66 18.98 2007-9-17 BH05 崩滑物源 12012 2.61 3.14 2007-9-17 BH06 崩滑物源 76095 3.99 30.39 2007-9-17 BH07 崩滑物源 483453 6.10 294.98 2013-10-9 BH08 崩滑物源 67118 3.88 26.04 2013-10-9 BH09 崩滑物源 126595 4.49 56.81 2013-10-9 BH10 崩滑物源 134525 4.55 61.21 2014-11-8 BH11 崩滑物源 5820 2.21 1.29 2017-12-4 BH12 崩滑物源 8658 2.43 2.10 2017-12-4 BH13 崩滑物源 7208 2.33 1.68 2018-12-26 BH14 崩滑物源 15242 2.76 4.21 2021-1-23 GD01 沟道堆积 4149 2.05 0.85 2007-9-17 GD02 沟道堆积 16190 2.80 4.53 2007-9-17 GD03 沟道堆积 47282 3.58 16.93 2014-11-8 GD04 沟道堆积 71395 3.94 28.09 2018-12-26
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